1. Field of the Invention
The present invention is directed to a multi-tone receiver and a system for operating the same.
2. Description of Related Art
The communication channel is the set of a physical medium, device and system that connects the transmitter to the receiver. The transmitter and receiver include an encoder and decoder, respectively, for translating the information stream produced by the source into a signal suitable for channel transmission, and vice versa. Some communications channels are impaired by impulsive noise. One way to maintain the relatively high integrity of the channels is to use an error correcting code, which restores the original data when corrupted by impulsive noise. A conventional encoding method for impulsive noise in a channel and used for asymmetrical digital subscriber line (ADSL) is described in the ITU G.992.1 standard, which is incorporated herein by reference. This particular scheme uses a Reed-Solomon encoder followed by a byte-interleaver. An ADSL decoder typically comprises a byte-deinterleaver, which distributes the bytes hit by impulsive noise between multiple Reed-Solomon code words, and a conventional Reed-Solomon decoder, which corrects the errors in each of the code words.
Correction capabilities or results of a Reed-Solomon decoder may be improved by using erasures to indicate when the reliability of the input bytes are corrupted. Applying this knowledge to a multi-tone system operating in an environment of impulsive noise, impulsive noise may be detected by measuring the accumulative error over one or more tones of a multi-tone symbol, since multiple tones will be impaired by the impulsive noise. Bytes of a symbol suspected to be corrupted or hit by impulsive noise may then be marked as erasures.
The use of erasures, however, is disadvantageous when no impulsive noise is present, since false alarm impulsive noise indicators may reduce the correction capabilities of the Reed-Solomon decoder. In addition, the detection threshold level is very limited. Specifically, on the one hand, a relatively high detection sensitivity may produce false alarms and reduce the performance when little, if any, impulsive noise is present; whereas, on the other hand, a relatively low detection sensitivity may result in missed detection of impulsive noise, thereby reducing the correction capabilities when impulsive noise is present.
It is therefore desirable to develop a system and method for use on a multi-tone symbol that exploits the advantages of using erasures when impulsive noise is present, without impairing the correction performance if no impulsive noise is present.
Impulsive noise is estimated for each discrete multi-tone (DMT) symbol. If impulsive noise is detected, all bytes, within the associated DMT symbol are tagged by xe2x80x9cerasure bitsxe2x80x9d. After interleaving, Reed-Solomon decoding is initially performed without erasures. If the decoding fails, it is performed again, this time with erasures. Reed-Solomon decoders report failure with relatively high certainty. Therefore, if the first stage (decoding without erasures) fails to decode into a proper codeword and impulsive noise is present, the second stage of decoding is performed again with erasures.
In a first embodiment, the multi-tone receiver in accordance with the present invention includes a decoder operable in one of two modes, a first mode without erasures for producing a first decoded data block and a second mode with erasures for producing a second decoded data block. The decoder generates a decoding-failure indicator when a decoding failure is detected. In addition, the receiver includes a controller which initiates the decoder to receive an input block of data; activates the decoder to operate in the first mode or the second mode based on the decoding-failure indicator reported by the decoder; and selects as an output from the decoder the first or second decoded data block output based on the decoding-failure indicator reported by the decoder.
The invention is also directed to a method for operating the multi-tone receiver described above. Initially, an input data block is decoded without erasures to produce a first decoded data block. A determination in then made whether a decoding-failure indicator is generated by the decoder. If a decoding-failure indicator is generated, the input data block is decoded with erasures to produce a second decoded data block. The decoded data block output from the decoder is selected between the first and second decoded data blocks based on a decoding-failure indicator. Alternatively, the input data block may be first decoded with erasures and then, decoded without erasures based on the decoding-failure indicator.
In a second embodiment, instead of using one decoder operable in two modes, the receiver may be designed with two parallel decoders, processing the same input bytes, simultaneously. The multi-tone receiver includes a first decoder for decoding without erasures a sample block to produce a first decoded data block, a second decoder for decoding with erasures the sample block to produce a second decoded data block, means for generating a decoding-failure indicator when a decoding failure is detected by one of said first and second decoders, and means for selecting between the first and second decoded data block based on the presence of a decoding-failure indicator.
The invention also relates to the method for operating the multi-tone receiver configured in accordance with the second embodiment of the invention. Initially, an input data block is decoded without erasures using a first decoder to produce a first decoded data block, and is decoded with erasures using the second decoder to produce a second decoded data block. A decoding-failure indicator is generated when a decoding failure is detected by the first or second decoders and a selection is made between the first and second decoded data block based on the presence of a decoding-failure indicator.